FMH QuikQuant™

Introduction

The most important clinical use of fetal RBC detection is the diagnosis and quantitation of fetomaternal hemorrhage (FMH). FMH occurs normally throughout pregnancy in minute amounts, with increasing volumes during the later stages of gestation. If there is a significant difference in the RBC antigenicity between the fetus and mother, this can result in allosensitization of the maternal immune system either before or after parturition. The maternal antibodies to the fetal RBC antigens may be clinically silent or cause life-threatening autoimmune sequelae for the current or subsequent pregnancies (e.g.: erythroblastosis fetalis or early abortion). Such sensitization can occur with any RBC antigen mismatch, but the highest frequency and profound clinical consequences occur with Rh or D-antigen mismatches.

Detection and enumeration of Fetal RBCs is an essential part of the management of those patients with FMH treated with Rh immune globulin (RhIG) preparations. The use of Rh immune globulin prophylaxis is a universal practice, but dosing amounts and schedules have regional variations. Hence, the sensitivity and specificity of detection assays for FMH is a critical factor in therapeutic efficacy and subsequent clinical outcome.

The most widely used assay for FMH detection has been the visual microscopic counting Kleihauer-Betke (KB) method, which is based upon the differences in solubility properties in acid conditions of fetal hemoglobin (HbF) from adult hemoglobin. While the KB method is easily performed by most clinical laboratories, it lacks sensitivity and exhibits poor reproducibility or precision (CVs of 50- 100%). Flow cytometric methods have been developed using the antigenic differences or quantitative assessment of fetal hemoglobin (HbF) to distinguish fetal RBCs from adult RBCs. These methods are more precise and less subjective. Nonetheless, many laboratories have continued to use the KB method due to the limited availability of Flow Cytometry.